Sheet processing apparatus and image forming system

ABSTRACT

A sheet processing apparatus includes a feed-in path through which a sheet is fed, a first transport path through which a sheet fed from the feed-in path is transported, and a second transport path through which a sheet fed from the feed-in path is transported. The sheet processing apparatus also includes the following elements. A first processor sequentially stacks sheets output from the first transport path and performs first processing on a bundle of stacked sheets. A second processor sequentially stacks sheets output from the second transport path and performs second processing, which is different from the first processing, on a bundle of stacked sheets. A common retention path causes a sheet fed through the feed-in path to temporarily remain on the common retention path and causes the sheet temporarily remaining on the common retention path to be output to one of the first and second transport paths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2012-119185 filed May 25, 2012.

BACKGROUND Technical Field

The present invention relates to a sheet processing apparatus and animage forming system.

SUMMARY

According to an aspect of the invention, there is provided a sheetprocessing apparatus including: a feed-in path through which a sheet isfed; a first transport path through which a sheet fed from the feed-inpath is transported; a first processor that sequentially stacks sheetsoutput from the first transport path and that performs first processingon a bundle of stacked sheets; a second transport path through which asheet fed from the feed-in path is transported; a second processor thatsequentially stacks sheets output from the second transport path andthat performs second processing, which is different from the firstprocessing, on a bundle of stacked sheets; and a common retention paththat causes a sheet fed through the feed-in path to temporarily remainon the common retention path and that causes the sheet temporarilyremaining on the common retention path to be output to one of the firsttransport path and the second transport path.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 illustrates an example of the entire configuration of an imageforming system of a first exemplary embodiment;

FIG. 2 illustrates a procedure of end-binding direct output processingin a post-processing apparatus of the first exemplary embodiment;

FIGS. 3A through 3D illustrate a procedure of end-binding indirectoutput processing in the post-processing apparatus of the firstexemplary embodiment;

FIG. 4 illustrates a procedure of saddle-stitch-binding direct outputprocessing in the post-processing apparatus of the first exemplaryembodiment;

FIGS. 5A through 5D illustrate a procedure of saddle-stitch-bindingindirect output processing in the post-processing apparatus of the firstexemplary embodiment;

FIG. 6 illustrates a procedure of non-binding output processing in thepost-processing apparatus of the first exemplary embodiment;

FIG. 7 is a timing chart illustrating an example of a procedure of endbinding processing;

FIG. 8 is a timing chart illustrating an example of a procedure ofsaddle stitch bookbinding processing;

FIG. 9 illustrates an example of the entire configuration of an imageforming system of a second exemplary embodiment;

FIG. 10 illustrates a procedure of end-binding direct output processingin a post-processing apparatus of the second exemplary embodiment;

FIGS. 11A through 11D illustrate a procedure of end-binding indirectoutput processing in the post-processing apparatus of the secondexemplary embodiment;

FIG. 12 illustrates a procedure of saddle-stitch-binding direct outputprocessing in the post-processing apparatus of the second exemplaryembodiment; and

FIGS. 13A through 13D illustrate a procedure of saddle-stitch-bindingindirect output processing in the post-processing apparatus of thesecond exemplary embodiment.

DETAILED DESCRIPTION

Exemplary embodiments of the invention will be described below in detailwith reference to the accompanying drawings.

First Exemplary Embodiment

FIG. 1 illustrates an example of the entire configuration of an imageforming system of a first exemplary embodiment.

The image forming system includes an image forming apparatus 1 whichforms an image on a sheet P and a post-processing apparatus 2 whichperforms post-processing on a sheet P on which an image is formed in theimage forming apparatus 1 and sent from the image forming apparatus 1.

The image forming apparatus 1 of the first exemplary embodiment forms animage on a sheet P by using an electrophotographic system. However, theimage forming apparatus 1 may form an image by using another system,such as an ink-jet system. Instead of using a direct-to-press printingsystem, such as an electrophotographic system or an ink-jet system, theimage forming apparatus 1 may use an offset printing system.Additionally, when continuously printing a file including plural pagesof image data items on plural sheets P and outputting them, the imageforming apparatus 1 may change the order in which images are formed andreverse the front and back sides of a sheet P (and the forward and rearends of the sheet P in the transport direction) after an image is formedon one side of the sheet P.

The post-processing apparatus 2, which is an example of a sheetprocessing apparatus, includes a sheet transport unit 10, an end bindingprocessor 30, and a saddle stitch binding processor 50. The sheettransport unit 10 transports sheets P sent from the image formingapparatus 1. The end binding processor 30 sequentially stacks sheets Ptransported by the sheet transport unit 10 and also performs end bindingprocessing on a bundle of stacked sheets P (hereinafter referred to as a“sheet bundle”). The saddle stitch binding processor 50 sequentiallystacks sheets P transported by the sheet transport unit 10 and alsoperforms saddle stitch binding processing on a sheet bundle. Thepost-processing apparatus 2 also includes first through third sheetstacking units 71 through 73, a controller 100, and a housing 2 a. Thefirst sheet stacking unit 71 stacks sheet bundles output from the endbinding processor 30. The second sheet stacking unit 72 stacks sheetbundles output from the saddle stitch binding processor 50. The thirdsheet stacking unit 73 stacks sheets P which are transported by thesheet transport unit 10 and which are output without being subjected toend binding processing or saddle stitch binding processing. Thecontroller 100 controls operations of the sheet transport unit 10, theend binding processor 30, and the saddle stitch binding processor 50.The housing 2 a houses the above-described elements of thepost-processing apparatus 2 therein. The controller 100 controls thepost-processing apparatus 2 together with a controller (not shown)provided in the image forming apparatus 1.

In the post-processing apparatus 2, the saddle stitch binding processor50 is positioned below the end binding processor 30. In thepost-processing apparatus 2, the second sheet stacking unit 72 ispositioned below the first sheet stacking unit 71, and the third sheetstacking unit 73 is positioned above the first sheet stacking unit 71.

In the post-processing apparatus 2, the sheet transport unit 10 includesa feed-in path R0, a first transport path R1, a second transport pathR2, a third transport path R3, a fourth transport path R4, and a fifthtransport path R5. The feed-in path R0 is connected to a sheet outputsection of the image forming apparatus 1 and receives sheets P from theimage forming apparatus 1. The first transport path R1 is connected tothe downstream end of the feed-in path R0 in the direction in whichsheets P are transported, and guides sheets P fed from the feed-in pathR0 to the end binding processor 30. The second transport path R2 isconnected to the downstream end of the feed-in path R0 and branches offfrom the feed-in path R0 in a direction different from that of the firsttransport path R1. The second transport path R2 guides sheets P fed fromthe feed-in path R0 to the saddle stitch binding processor 50. The thirdtransport path R3 branches off from a midway portion of the firsttransport path R1 and guides sheets P fed from the feed-in path R0 andthe first transport path R1 to the third sheet stacking unit 73. Thefourth transport path R4, which is an example of a common retentionpath, branches off from the upstream side of the second transport pathR2 and joins the downstream side of the second transport path R2. Thefifth transport path R5, which is an example of a connection transportpath, branches off from a midway portion of the fourth transport path R4and joins a midway portion of the first transport path R1. In the firstexemplary embodiment, the second transport path R2 is positioned belowthe first transport path R1, and the third transport path R3 ispositioned above the first transport path R1.

The sheet transport unit 10 includes first through ninth transportrollers 11 through 19. The first transport rollers 11 are provided onthe farther upstream side than the portion at which the third transportpath R3 branches off from the first transport path R1 in the transportdirection of sheets P. The second transport rollers 12 are provided onthe most downstream side of the first transport path R1 in the transportdirection of sheets P. The third transport rollers 13 are provided onthe farther upstream side than the portion at which the fourth transportpath R4 branches off from the second transport path R2 in the transportdirection of sheets P. The fourth transport rollers 14 are provided onthe farther downstream side than the portion at which the fourthtransport path R4 branches off from the second transport path R2 in thetransport direction of sheets P and on the upstream side than theportion at which the fourth transport path R4 joins the second transportpath R2 in the transport direction of sheets P. The fifth transportrollers 15 are provided on the most downstream side of the secondtransport path R2 in the transport direction of sheets P. The sixthtransport rollers 16 are provided on the third transport path R3. Theseventh transport rollers 17 are provided on the most downstream side ofthe third transport path R3 in the transport direction of sheets P. Theeighth transport rollers 18 are provided on the fourth transport pathR4. The ninth transport rollers 19 are provided on the fifth transportpath R5.

The sheet transport unit 10 also includes first through third gates 21through 23. The first gate 21 is positioned at a portion at which thesecond transport path R2 branches off from the first transport path R1.The second gate 22 is positioned at a portion at which the thirdtransport path R3 branches off from the first transport path R1. Thethird gate 23 is positioned at a portion at which the fourth transportpath R4 branches off from the second transport path R2. The first gate21 is rotatable around a shaft (not shown) and is located at a positionat which it can feed a sheet P supplied from the feed-in path R0 to oneof the first and second transport paths R1 and R2. The second gate 22 isalso rotatable around a shaft (not shown) and is located at a positionat which it can feed a sheet P supplied from the upstream side of thefirst transport path R1 to one of the downstream side of the firsttransport path R1 and the third transport path R3. The third gate 23 isalso rotatable around a shaft (not shown) and is located at a positionat which it can feed a sheet P supplied from the upstream side of thesecond transport path R2 to one of the downstream side of the secondtransport path R2 and the fourth transport path R4. The third gate 23also has a function of blocking the fourth transport path R4 at theportion at which the fifth transport path R5 branches off from thefourth transport path R4 when feeding a sheet P from the fourthtransport path R4 to the fifth transport path R5.

In the post-processing apparatus 2, the end binding processor 30, whichis an example of a first processor, includes an end-binding sheetstacking section 31, an end-binding sheet abutting section 32, a firstend-binding paddle 33, a second end-binding paddle 34, an end bindingstapler 35, and end binding output rollers 36. Sheets P output from thefirst transport path R1 through the second transport rollers 12 aresequentially stacked in the end-binding sheet stacking section 31. Oneend of each of sheets P stacked in the end-binding sheet stackingsection (rear end of each of sheets P output from the first transportpath R1 in the transport direction) abuts against the end-binding sheetabutting section 32. The first and second end-binding paddles 33 and 34are rotatable and transport sheets P stacked in the end-binding sheetstacking section 31 to the end-binding sheet abutting section 32. Theend binding stapler 35 is located adjacent to the end-binding sheetabutting section 32 and performs binding processing by stapling one end(which is near the end-binding sheet abutting section 32) of a sheetbundle stacked in the end-binding sheet stacking section 31. The endbinding output rollers 36 output a sheet bundle stacked in theend-binding sheet stacking section 31 to the first sheet stacking unit71. The end binding output rollers 36 include a first roller disposedbelow the end-binding sheet stacking section 31 and a second rollerwhich is disposed above the end-binding sheet stacking section 31 andwhich is movable close to and away from the first roller. The endbinding processor 30 also includes a tamper (not shown) which aligns thewidth direction (which intersects with the transport direction) ofsheets P stacked in the end-binding sheet stacking section 31.

In the post-processing apparatus 2, the saddle stitch binding processor50, which is an example of a second processor, includes asaddle-stitch-binding sheet stacking section 51, a saddle-stitch-bindingsheet abutting section 52, a saddle stitch binding paddle 53, a saddlestitch binding stapler 54, a folding processor 55, and a saddle stitchbinding output roller 56. Sheets P output from the second transport pathR2 through the fifth transport rollers 15 are sequentially stacked inthe saddle-stitch-binding sheet stacking section 51. One end of each ofsheets P stacked in the saddle-stitch-binding sheet stacking section(rear end of each of sheets P output from the second transport path R2in the transport direction) abuts against the saddle-stitch-bindingsheet abutting section 52. The saddle stitch binding paddle 53 isrotatable and transports sheets P stacked in the saddle-stitch-bindingsheet stacking section 51 to the saddle-stitch-binding sheet abuttingsection 52. The saddle stitch binding stapler 54 opposes thesaddle-stitch-binding sheet stacking section 51 and performs bindingprocessing by stapling a central portion (in the transport direction) ofa sheet bundle stacked in the saddle-stitch-binding sheet stackingsection 51. The folding processor 55 performs two-folding processing forfolding a sheet bundle, which has been subjected to binding processingby using the saddle stitch binding stapler 54, into two with respect tothe central portion of the sheet bundle. The saddle stitch bindingoutput rollers 56 output a sheet bundle subjected to binding processingand two-folding processing to the second sheet stacking unit 72. Thesaddle stitch binding processor 50 also includes a tamper (not shown)which aligns the width direction (which intersect with the transportdirection) of sheets P stacked in the saddle-stitch-binding sheetstacking section 51. As described above, the saddle stitch bindingprocessor 50 has a function of binding and outputting a sheet bundle(saddle stitch binding function) by performing binding processing bystapling a central portion of the sheet bundle and by performingtwo-folding processing for folding the sheet bundle with respect to thecentral portion.

In the post-processing apparatus 2 of the first exemplary embodiment,one of the “end binding processing”, which is an example of firstprocessing, “saddle stitch bookbinding processing”, which is an exampleof second processing, and “non-binding processing” is selectivelyperformed. In the end binding processing, the sheet transport unit 10sequentially supplies sheets P fed from the image forming apparatus 1 tothe end binding processor 30, and the end binding processor 30 performsend binding processing on a sheet bundle constituted by the sheets P andoutputs the sheet bundle to the first sheet stacking unit 71. In thesaddle stitch binding processing, the sheet transport unit 10sequentially supplies sheets P fed from the image forming apparatus 1 tothe saddle stitch binding processor 50, and the saddle stitch bindingprocessor 50 performs saddle stitch binding processing and two-foldingprocessing on a sheet bundle constituted by the sheets P and outputs thesheet bundle to the second sheet stacking unit 72. In the non-bindingprocessing, the sheet transport unit 10 outputs sheets P fed from theimage forming apparatus 1 to the third sheet stacking unit 73 withoutperforming any processing.

In the end binding processing, one of “end-binding direct outputprocessing” and “end-binding indirect output processing” is performed.In the end-binding direct output processing, sheets P are output fromthe feed-in path R0 to the end binding processor 30 via the firsttransport path R1. In the end-binding indirect output processing, sheetsP are output from the feed-in path R0 to the end binding processor 30via the second transport path R2, the fourth transport path R4, thefifth transport path R5, and the first transport path R1.

In the saddle stitch bookbinding processing, one of“saddle-stitch-binding direct output processing” and“saddle-stitch-binding indirect output processing” is performed. In thesaddle-stitch-binding direct output processing, sheets P are output fromthe feed-in path R0 to the saddle stitch binding processor 50 via thesecond transport path R2. In the saddle-stitch-binding indirect outputprocessing, sheets P are output from the feed-in path R0 to the saddlestitch binding processor 50 via the second transport path R2, the fourthtransport path R4, and the second transport path R2.

In the non-binding processing, “non-binding output processing” isperformed in which sheets P are output from the feed-in path R0 to thethird sheet stacking unit 73 via the first transport path R1 and thethird transport path R3.

Procedures for transporting sheets P in the above-described end-bindingdirect output processing, end-binding indirect output processing,saddle-stitch-binding direct output processing, saddle-stitch-bindingindirect output processing, and non-binding output processing will bedescribed below in this order.

FIG. 2 illustrates a procedure of the end-binding direct outputprocessing in the post-processing apparatus 2 of the first exemplaryembodiment.

In the end-binding direct output processing, the first gate 21 islocated at a position at which the feed-in path R0 and the firsttransport path R1 are connected (the feed-in path R0 and the secondtransport path R2 are disconnected), and the second gate 22 is locatedat a position at which the upstream side and the downstream side of thefirst transport path R1 are connected (the upstream side of the firsttransport path R1 and the third transport path R3 are disconnected). Inthe end-binding direct output processing, the sheet P does not passthrough the mounting position of the third gate 23.

In the end-binding direct output processing, the sheet P supplied fromthe feed-in path R0 is output to the end binding processor 30 via thefirst transport rollers 11 and the second transport rollers 12 providedon the first transport path R1, as indicated by the thick arrow in FIG.2.

FIGS. 3A through 3D illustrate a procedure of the end-binding indirectoutput processing in the post-processing apparatus 2 of the firstexemplary embodiment.

FIG. 3A illustrates a first state (the state in which a sheet P is fedfrom the image forming apparatus 1 via the feed-in path R0) of theend-binding indirect output processing. In the first state, the firstgate 21 is located at a position at which the feed-in path R0 and thesecond transport path R2 are connected (the feed-in path R0 and thefirst transport path R1 are disconnected), and the third gate 23 islocated at a position at which the upstream side of the second transportpath R2 and the fourth transport path R4 are connected (the upstreamside and the downstream side of the second transport path R2 aredisconnected). In the end-binding indirect output processing, the sheetP does not pass through the mounting position of the second gate 22.

In the first state, the sheet P fed from the image forming apparatus 1via the feed-in path R0 is input into the fourth transport path R4 viathe second transport path R2 through the third transport rollers 13provided on the second transport path R2 and the eighth transportrollers 18 provided on the fourth transport path R4, as indicated by thethick arrow in FIG. 3A. Then, after the rear end of the sheet P passesthrough the portion at which the fifth transport path R5 branches offfrom the fourth transport path R4, the rotation of the eighth transportrollers 18 is stopped.

FIG. 3B illustrates a second state (the state in which the sheet Premains in the fourth transport path R4) after the first state shown inFIG. 3A in the end-binding indirect output processing. In the secondstate, both of the forward and rear ends of the sheet P in the transportdirection are located in the fourth transport path R4.

FIG. 3C illustrates a third state (the state in which the sheet P is fedout from the fourth transport path R4) after the second state shown inFIG. 3B in the end-binding indirect output processing. In the thirdstate, the third gate 23 is located at a position at which thedownstream side of the fourth transport path R4 and the fifth transportpath R5 are connected (the downstream side and the upstream side of thefourth transport path R4 are disconnected).

In the third state, the eighth transport rollers 18 provided on thefourth transport path R4 are rotated in a direction opposite to thedirection in which the sheet P is fed into the post-processing apparatus2, thereby causing the sheet P to be transported in a direction oppositeto the direction in which the sheet P is fed into the post-processingapparatus 2. Accordingly, the sheet P is transported from the fourthtransport path R4 to the fifth transport path R5, as indicated by thethick arrow in FIG. 3C, and are output to the end binding processor 30via the ninth transport rollers 19 provided on the fifth transport pathR5 and the second transport rollers 12 provided on the first transportpath R1.

In the example shown in FIG. 3C, the sheet P which is transported to thefourth transport path R4 is singly output to the end binding processor30. However, as shown in FIG. 3D, in accordance with the timing at whichthe next sheet P (indicated by the long dashed dotted arrow in FIG. 3D)is subjected to the end-binding direct output processing through thefirst transport path R1, the sheet P (indicated by the solid arrow inFIG. 3D) transported via the fourth transport path R4 may be output tothe end binding processor 30. In this case, two sheets P are output tothe end binding processor 30 in the state in which they are superposedon each other.

FIG. 4 illustrates a procedure of saddle-stitch-binding direct outputprocessing in the post-processing apparatus 2 of the first exemplaryembodiment.

In the saddle-stitch-binding direct output processing, the first gate 21is located at a position at which the feed-in path R0 and the secondtransport path R2 are connected (the feed-in path R0 and the firsttransport path R1 are disconnected), and the third gate 23 is located ata position at which the upstream side and the downstream side of thesecond transport path R2 are connected (the upstream side of the secondtransport path R2 and the fourth transport path R4 are disconnected). Inthe saddle-stitch-binding direct output processing, the sheet P does notpass through the mounting position of the second gate 22.

In the saddle-stitch-binding direct output processing, the sheet P fedfrom the feed-in path R0 is output to the saddle stitch bindingprocessor 50 via the third transport rollers 13, the fourth transportrollers 14, and the fifth transport rollers 15 provided on the secondtransport path R2.

FIGS. 5A through 5D illustrate a procedure of saddle-stitch-bindingindirect output processing in the post-processing apparatus 2 of thefirst exemplary embodiment.

FIG. 5A illustrates a first state (the state in which a sheet P is fedfrom the image forming apparatus 1 via the feed-in path R0) of thesaddle-stitch-binding indirect output processing. In the first state,the first gate 21 is located at a position at which the feed-in path R0and the second transport path R2 are connected (the feed-in path R0 andthe first transport path R1 are disconnected), and the third gate 23 islocated at a position at which the upstream side of the second transportpath R2 and the fourth transport path R4 are connected (the upstreamside and the downstream side of the second transport path R2 aredisconnected). In the saddle-stitch-binding indirect output processing,the sheet P does not pass through the mounting position of the secondgate 22.

In the first state, the sheet P fed from the image forming apparatus 1through the feed-in path R0 is input into the fourth transport path R4via the second transport path R2 through the third transport rollers 13provided on the second transport path R2 and the eighth transportrollers 18 provided on the fourth transport path R4, as indicated by thethick arrow in FIG. 5A. Then, after the rear end of the sheet P passesthrough the portion at which the fourth transport path R4 branches offfrom the second transport path R2, the rotation of the eighth transportrollers 18 is stopped.

FIG. 5B illustrates a second state (the state in which the sheet Premains in the fourth transport path R4) after the first state shown inFIG. 5A in the saddle-stitch-binding indirect output processing. In thesecond state, both of the forward and rear ends of the sheet P in thetransport direction are located in the fourth transport path R4.

FIG. 5C illustrates a third state (the state in which the sheet P is fedout from the fourth transport path R4) after the second state shown inFIG. 5B in the saddle-stitch-binding indirect output processing.

In the third state, the eighth transport rollers 18 provided on thefourth transport path R4 are rotated in the same direction as that whenthe sheet P is fed into the post-processing apparatus 2, thereby causingthe sheet P to be transported in the same direction as that when thesheet P is fed into the post-processing apparatus 2. Accordingly, thesheet P is transported from the fourth transport path R4 to the secondtransport path R2, as indicated by the thick arrow in FIG. 5C, and isoutput to the saddle stitch binding processor 50 via the fifth transportrollers 15 provided on the second transport path R2.

In the example shown in FIG. 5C, the sheet P which is transported to thefourth transport path R4 is singly output to the saddle stitch bindingprocessor 50. However, as shown in FIG. 5D, in accordance with thetiming at which the next sheet P (indicated by the long dashed dottedarrow) is subjected to saddle-stitch-binding direct output processingthrough the second transport path R2, the sheet P (indicated by thesolid arrow in FIG. 5D) transported via the fourth transport path R4 maybe output to the saddle stitch binding processor 50. In this case, twosheets P are output to the saddle stitch binding processor 50 in thestate in which they are superposed on each other.

FIG. 6 illustrates a procedure of non-binding output processing in thepost-processing apparatus 2 of the first exemplary embodiment.

In the non-binding output processing, the first gate 21 is located at aposition at which the feed-in path R0 and the first transport path R1are connected (the feed-in path R0 and the second transport path R2 aredisconnected), and the second gate 22 is located at a position at whichthe upstream side of the first transport path R1 and the third transportpath R3 are connected (the upstream side and the downstream side of thefirst transport path R1 are disconnected). In the non-binding outputprocessing, the sheet P does not pass through the mounting position ofthe third gate 23.

In the non-binding output processing, the sheet P supplied through thefeed-in path R0 is output to the third sheet stacking unit 73 via thefirst transport rollers 11 provided on the first transport path R1 andthe sixth and seventh transport rollers 16 and 17 provided on the thirdtransport path R3.

End-binding processing and saddle stitch bookbinding processingperformed by the post-processing apparatus 2 of the first exemplaryembodiment will be each described below through specific examples.

FIG. 7 is a timing chart illustrating an example of a procedure of endbinding processing. In FIG. 7, the top section indicates sheets Ppassing through the feed-in path R0, and the bottom section indicatesthe number of sheets P stacked in the end binding processor 30 (and morespecifically, the end-binding sheet stacking section 31). In thisexample, it is assumed that three sheets P form one sheet bundle andthat each sheet bundle is subjected to end binding processing and isoutput.

The first sheet P (indicated by (1) in FIG. 7, and the first sheets ofother sheet bundles are also indicated by (1) in FIGS. 7 and 8) forminga first sheet bundle passes through the feed-in path R0. The first sheetP is output to the end binding processor 30 after being subjected to theabove-described end-binding direct output processing (indicated by“direct” in FIG. 7, and this operation for other sheet bundles is alsoindicated by “direct” in FIGS. 7 and 8), and is stacked in theend-binding sheet stacking section 31.

Then, the second sheet P (indicated by (2) in FIG. 7, and the secondsheets of other sheet bundles are also indicated by (2) in FIGS. 7 and8) forming the first sheet bundle passes through the feed-in path R0.The second sheet P is also output to the end binding processor 30 afterbeing subjected to the end-binding direct output processing, and isstacked in the end-binding sheet stacking section 31 on top of the firstsheet P.

Then, the third sheet P (indicated by (3) in FIG. 7, and the thirdsheets of other sheet bundles are also indicated by (3) in FIGS. 7 and8) forming the third sheet bundle passes through the feed-in path R0.The third sheet P is also output to the end binding processor 30 afterbeing subjected to the end-binding direct output processing, and isstacked in the end-binding sheet stacking section 31 on top of the firstand second sheets P.

Then, after the three sheets P, which form the first sheet bundle, arestacked in the end-binding sheet stacking section 31, they are subjectedto the end binding processing by using the end binding stapler 35, andare then output to the first sheet stacking unit 71 through the endbinding output rollers 36 (indicated by “end-binding/output” in FIG. 7,and this operation for the second and third sheet bundles is alsoexpressed in the same way).

While the first sheet bundle is being subjected to the end bindingprocessing, the first sheet P forming the second sheet bundle passesthrough the feed-in path R0. The first sheet P temporarily enters astandby state in the fourth transport path R4 in accordance with theabove-described end-binding indirect output processing (indicated by“indirect” in FIG. 7, and this operation for other sheet bundles is alsoindicated by “indirect” in FIGS. 7 and 8). Subsequently, the secondsheet P forming the second sheet bundle passes through the feed-in pathR0. The second sheet P is transported in accordance with the end-bindingdirect output processing and is output to the end binding processor 30together with the first sheet P which is supplied through the fourthtransport path R4 in accordance with the end-binding indirect outputprocessing (see FIG. 3D). With this operation, the first and secondsheets P are stacked in the end-binding sheet stacking section 31 on topof each other.

The third sheet P forming the second sheet bundle passes through thefeed-in path R0. The third sheet P is also output to the end bindingprocessor 30 in accordance with the end-binding direct output processingand is stacked in the end-binding sheet stacking section 31 on top ofthe first and second sheets P.

Then, after the three sheets P forming the second sheet bundle arestacked in the end-binding sheet stacking section 31, they are subjectedto end binding processing by using the end binding stapler 35 and areoutput to the first sheet stacking unit 71 through the end bindingoutput rollers 36.

While the second sheet bundle is being subjected to the end bindingprocessing, the first sheet P forming the third sheet bundle passesthrough the feed-in path R0. The first sheet P temporarily enters astandby state in the fourth transport path R4 in accordance with theend-binding indirect output processing. Subsequently, the second sheet Pforming the third sheet bundle passes through the feed-in path R0. Thesecond sheet P is transported in accordance with the end-binding directoutput processing and is output to the end binding processor 30 togetherwith the first sheet P which is supplied through the fourth transportpath R4 in accordance with the end-binding indirect output processing(see FIG. 3D). With this operation, the first and second sheets P arestacked in the end-binding sheet stacking section 31 on top of eachother.

The third sheet P forming the third sheet bundle passes through thefeed-in path R0. The third sheet P is also output to the end bindingprocessor 30 in accordance with the end-binding direct output processingand is stacked in the end-binding sheet stacking section 31 on top ofthe first and second sheets P.

Then, after the three sheets P, which forms the third sheet bundle, arestacked in the end-binding sheet stacking section 31, they are subjectedto end binding processing by using the end binding stapler 35 and arethen output to the first sheet stacking unit 71 via the end bindingoutput rollers 36.

Thereafter, by repeating this procedure, forming three sheets P into asheet bundle, performing end binding processing on a formed sheetbundle, and outputting a sheet bundle subjected to the end bindingprocessing are sequentially performed.

In this procedure, if the first sheet P in the second or subsequentsheet bundle were transported to the end binding processor 30 inaccordance with the end-binding direct output processing, it wouldcollide against the previous sheet bundle since the previous sheetbundle is being subjected to end binding processing in the end bindingprocessor 30.

In contrast, in the first exemplary embodiment, the first sheets P ofthe second and subsequent sheet bundles are transported to the endbinding processor 30 in accordance with the end-binding indirect outputprocessing, thereby decreasing the possibility of such a collisionoccurring. Additionally, in the first exemplary embodiment, since thefirst and second sheets P of each of the second and subsequent sheetbundles are transported to the end binding processor 30 together, theinterval before supplying the third sheet P does not have to beincreased.

FIG. 8 is a timing chart illustrating an example of a procedure ofsaddle stitch bookbinding processing. In FIG. 8, the top sectionindicates sheets P passing through the feed-in path R0, and the bottomsection indicates the number of sheets P stacked in the saddle stitchbinding processor (and more specifically, the saddle-stitch-bindingsheet stacking section 51). In this example, it is assumed that threesheets P form one sheet bundle and that each sheet bundle is subjectedto saddle stitch bookbinding processing and is output.

The first sheet P forming a first sheet bundle passes through thefeed-in path R0. The first sheet P is output to the saddle stitchbinding processor 50 after being subjected to the above-describedsaddle-stitch-binding direct output processing, and is stacked in thesaddle-stitch-binding sheet stacking section 51.

Then, the second sheet P forming the first sheet bundle passes throughthe feed-in path R0. The second sheet P is also output to the saddlestitch binding processor 50 after being subjected to thesaddle-stitch-binding direct output processing, and is stacked in thesaddle-stitch-binding sheet stacking section 51 on top of the firstsheet P.

Then, the third sheet P forming the third sheet bundle passes throughthe feed-in path R0. The third sheet P is also output to the saddlestitch binding processor 50 after being subjected to thesaddle-stitch-binding direct output processing, and is stacked in thesaddle-stitch-binding sheet stacking section 51 on top of the first andsecond sheets P.

Then, after the three sheets P, which form the first sheet bundle, arestacked in the saddle-stitch-binding sheet stacking section 51, they aresubjected to saddle stitch binding processing by using the saddle stitchbinding stapler 54 and two-folding processing by using the foldingprocessor 55, and are then output to the second sheet stacking unit 72through the saddle stitch binding output rollers 56 (indicated by“saddle-stitch-binding/two-folding/output” in FIG. 8, and this operationfor the second and third sheet bundles is also expressed in the sameway).

While the first sheet bundle is being subjected to saddle stitch bindingprocessing and two-folding processing, the first sheet P forming thesecond sheet bundle passes through the feed-in path R0. The first sheetP temporarily enters a standby state in the fourth transport path R4 inaccordance with the above-described saddle-stitch-binding indirectoutput processing. Subsequently, the second sheet P forming the secondsheet bundle passes through the feed-in path R0. The second sheet P istransported in accordance with the saddle-stitch-binding direct outputprocessing and is output to the saddle stitch binding processor 50together with the first sheet P which is supplied through the fourthtransport path R4 in accordance with the saddle-stitch-binding indirectoutput processing (see FIG. 5D). With this operation, the first andsecond sheets P are stacked in the saddle-stitch-binding sheet stackingsection 51 on top of each other.

The third sheet P forming the second sheet bundle passes through thefeed-in path R0. The third sheet P is also output to the saddle stitchbinding processor 50 in accordance with the saddle-stitch-binding directoutput processing and is stacked in the saddle-stitch-binding sheetstacking section 51 on top of the first and second sheets P.

Then, after the three sheets P forming the second sheet bundle arestacked in the saddle-stitch-binding sheet stacking section 51, they aresubjected to saddle stitch binding processing by using the saddle stitchbinding stapler 54 and two-folding processing by using the foldingprocessor 55 and are output to the second sheet stacking unit 72 throughthe saddle stitch binding output rollers 56.

While the second sheet bundle is being subjected to saddle stitchbinding processing and two-folding processing, the first sheet P formingthe third sheet bundle passes through the feed-in path R0. The firstsheet P temporarily enters a standby state in the fourth transport pathR4 in accordance with the saddle-stitch-binding indirect outputprocessing. Subsequently, the second sheet P forming the third sheetbundle passes through the feed-in path R0. The second sheet P istransported in accordance with the saddle-stitch-binding direct outputprocessing and is output to the saddle stitch binding processor 50together with the first sheet P which is supplied through the fourthtransport path R4 in accordance with the saddle-stitch-binding indirectoutput processing (see FIG. 5D). With this operation, the first andsecond sheets P are stacked in the saddle-stitch-binding sheet stackingsection 51 on top of each other.

The third sheet P forming the third sheet bundle passes through thefeed-in path R0. The third sheet P is also output to the saddle stitchbinding processor 50 in accordance with the saddle-stitch-binding directoutput processing and is stacked in the saddle-stitch-binding sheetstacking section 51 on top of the first and second sheets P.

Then, after the three sheets P, which forms the third sheet bundle, arestacked in the saddle-stitch-binding sheet stacking section 51, they aresubjected to saddle stitch binding processing by using the saddle stitchbinding stapler 54 and two-folding processing by using the foldingprocessor 55 and are then output to the second sheet stacking unit 72via the saddle stitch binding output rollers 56.

Thereafter, by repeating this procedure, forming three sheets P into asheet bundle, performing saddle stitch binding processing andtwo-folding processing on a formed sheet bundle, and outputting a sheetbundle subjected to the saddle stitch binding processing and two-foldingprocessing are sequentially performed.

If the first sheet P in the second or subsequent sheet bundle weretransported to the saddle stitch binding processor 50 in accordance withthe saddle-stitch-binding direct output processing, it would collideagainst the previous sheet bundle since the previous sheet bundle isbeing subjected to saddle stitch binding processing and two-foldingprocessing in the saddle stitch binding processor 50.

In contrast, in the first exemplary embodiment, the first sheets P ofthe second and subsequent sheet bundles are transported to the saddlestitch binding processor 50 in accordance with the saddle-stitch-bindingindirect output processing, thereby decreasing the possibility of such acollision occurring. Additionally, in the first exemplary embodiment,since the first and second sheets P of each of the second and subsequentsheet bundles are transported to the saddle stitch binding processor 50together, the interval before supplying the third sheet P does not haveto be increased.

In the first exemplary embodiment, the fourth transport path R4 is usedboth as a transport path on which a sheet P temporarily remains in astandby state during the end-binding indirect output processing beforebeing output to the end binding processor 30 and as a transport path onwhich a sheet P temporarily remains in a standby state during thesaddle-stitch-binding indirect output processing before being output tothe saddle stitch binding processor 50. Thus, the size of thepost-processing apparatus 2 does not have to be increased, unlike a casein which a transport path on which a sheet P remains in a standby stateduring the end-binding indirect output processing and that during thesaddle-stitch-binding indirect output processing are separatelyprovided.

Additionally, in the first exemplary embodiment, the fourth transportpath R4 is used both for end binding processing and saddle stitchbookbinding processing. Accordingly, the length of a sheet P (in thetransport direction) subjected to the end binding processing is equal tothat subjected to the saddle stitch bookbinding processing. That is, inthe end binding processing, a sheet P having the same length as that ofa sheet P subjected to the saddle stitch bookbinding processing may beused.

Second Exemplary Embodiment

The basic configuration of a second exemplary embodiment is similar tothat of the first exemplary embodiment. However, the configuration of asheet transport unit 10 of a post-processing apparatus 2 of the secondexemplary embodiment is partially different from that of the firstexemplary embodiment. In the second exemplary embodiment, elementssimilar to those of the first exemplary embodiment are designated bylike reference numerals, and an explanation thereof will thus beomitted.

FIG. 9 illustrates an example of the entire configuration of an imageforming system of the second exemplary embodiment.

The image forming system includes an image forming apparatus 1 whichforms an image on a sheet P and a post-processing apparatus 2 whichperforms post-processing on a sheet P on which an image is formed in theimage forming apparatus 1 and sent from the image forming apparatus 1.The post-processing apparatus 2 includes a sheet transport unit 10, anend binding processor 30, a saddle stitch binding processor 50, firstthrough third sheet stacking units 71 through 73, a controller 100, anda housing 2 a.

The post-processing apparatus 2 of the second exemplary embodimentdiffers from that of the first exemplary embodiment in the followingpoints. The fifth transport path R5 provided in the sheet transport unit10 does not branch off from a midway portion of the fourth transportpath R4 but branches off from the second transport path R2 at a positionon the farther downstream side in the transport direction than a portionat which the second transport path R2 and the fourth transport path R4join each other. The sheet transport unit 10 also includes a fourth gate24 at a portion at which the fifth transport path R5 branches off fromthe second transport path R2. The fourth gate 24 is rotatable around ashaft (not shown) and is located at a position at which it can feed asheet P supplied from the upstream side of the second transport path R2to one of the downstream side of the second transport path R2 and thefifth transport path R5. The sheet transport unit 10 also includesplural eighth transport rollers 18 provided on the fourth transport pathR4 and plural ninth transport rollers 19 provided on the fifth transportpath R5.

In the post-processing apparatus 2 of the second exemplary embodiment,as well as that of the first exemplary embodiment, one of theabove-described end binding processing, saddle stitch bookbindingprocessing, and non-binding processing is selectively performed. In theend binding processing, one of the end-binding direct output processingand end-binding indirect output processing is performed. In the saddlestitch bookbinding processing, one of the saddle-stitch-binding directoutput processing and saddle-stitch-binding indirect output processingis performed. In the non-binding processing, non-binding outputprocessing is performed.

Procedures for transporting sheets P in the above-described end-bindingdirect output processing, end-binding indirect output processing,saddle-stitch-binding direct output processing, andsaddle-stitch-binding indirect output processing will be described belowin this order. Concerning the non-binding output processing, the sameprocedure as that of the first exemplary embodiment is taken, and adetailed explanation thereof will thus be omitted.

FIG. 10 illustrates a procedure of end-binding direct output processingin the post-processing apparatus 2 of the second exemplary embodiment.

In the end-binding direct output processing, the first gate 21 islocated at a position at which the feed-in path R0 and the firsttransport path R1 are connected (the feed-in path R0 and the secondtransport path R2 are disconnected), and the second gate 22 is locatedat a position at which the upstream side and the downstream side of thefirst transport path R1 are connected (the upstream side of the firsttransport path R1 and the third transport path R3 are disconnected). Inthe end-binding direct output processing, the sheet P does not passthrough the mounting position of the third gate 23 or the fourth gate24.

In the end-binding direct output processing, the sheet P supplied fromthe feed-in path R0 is output to the end binding processor 30 via thefirst transport rollers 11 and the second transport rollers 12 providedon the first transport path R1, as indicated by the thick arrow in FIG.10.

FIGS. 11A through 11D illustrate a procedure of end-binding indirectoutput processing in the post-processing apparatus 2 of the secondexemplary embodiment.

FIG. 11A illustrates a first state (the state in which a sheet P is fedfrom the image forming apparatus 1 via the feed-in path R0) of theend-binding indirect output processing. In the first state, the firstgate 21 is located at a position at which the feed-in path R0 and thesecond transport path R2 are connected (the feed-in path R0 and thefirst transport path R1 are disconnected), the third gate 23 is locatedat a position at which the upstream side of the second transport path R2and the fourth transport path R4 are connected (the upstream side andthe downstream side of the second transport path R2 are disconnected),and the fourth gate 24 is located at a position at which the upstreamside of the second transport path R2 and the fifth transport path R5 areconnected (the upstream side and the downstream side of the secondtransport path R2 are disconnected). In the end-binding indirect outputprocessing, the sheet P does not pass through the mounting position ofthe second gate 22.

In the first state, the sheet P fed from the image forming apparatus 1via the feed-in path R0 is input into the fourth transport path R4 viathe second transport path R2 through the third transport rollers 13provided on the second transport path R2 and the eighth transportrollers 18 provided on the fourth transport path R4, as indicated by thethick arrow in FIG. 11A. Then, after the rear end of the sheet P in thetransport direction passes through the portion at which the fourthtransport path R4 branches off from the second transport path R2, therotation of the eighth transport rollers 18 is stopped.

FIG. 11B illustrates a second state (the state in which the sheet Premains in the fourth transport path R4) after the first state shown inFIG. 11A in the end-binding indirect output processing. In the secondstate, both of the forward and rear ends of the sheet P in the transportdirection are located in the fourth transport path R4.

FIG. 11C illustrates a third state (the state in which the sheet P isfed out from the fourth transport path R4) after the second state shownin FIG. 11B in the end-binding indirect output processing.

In the third state, the eighth transport rollers 18 provided on thefourth transport path R4 are rotated in the same direction as that whenthe sheet P is fed into the post-processing apparatus 2, thereby causingthe sheet P to be transported in the same direction as that when thesheet P is fed into the post-processing apparatus 2. Accordingly, thesheet P is transported from the fourth transport path R4 to the fifthtransport path R5 via the second transport path R2, as indicated by thethick arrow in FIG. 11C, and is output to the end binding processor 30via the ninth transport rollers 19 provided on the fifth transport pathR5 and the second transport rollers 12 provided on the first transportpath R1.

In the example shown in FIG. 11C, the sheet P which is transported tothe fourth transport path R4 is singly output to the end bindingprocessor 30. However, as shown in FIG. 11D, in accordance with thetiming at which the next sheet P (indicated by the long dashed dottedarrow in FIG. 11D) is subjected to end-binding direct output processingthrough the first transport path R1, the sheet P (indicated by the solidarrow in FIG. 11D) transported via the fourth transport path R4 may beoutput to the end binding processor 30. In this case, two sheets P areoutput to the end binding processor 30 in the state in which they aresuperposed on each other.

FIG. 12 illustrates a procedure of saddle-stitch-binding direct outputprocessing in the post-processing apparatus 2 of the second exemplaryembodiment.

In the saddle-stitch-binding direct output processing, the first gate 21is located at a position at which the feed-in path R0 and the secondtransport path R2 are connected (the feed-in path R0 and the firsttransport path R1 are disconnected), and the third gate 23 is located ata position at which the upstream side and the downstream side of thesecond transport path R2 are connected (the upstream side of the secondtransport path R2 and the fourth transport path R4 are disconnected).The fourth gate 24 is located at a position at which the upstream sideand the downstream side of the second transport path R2 are connected(the upstream side of the second transport path R2 and the fifthtransport path R5 are disconnected). In the saddle-stitch-binding directoutput processing, the sheet P does not pass through the mountingposition of the second gate 22.

In the saddle-stitch-binding direct output processing, the sheet P fedfrom the feed-in path R0 is output to the saddle stitch bindingprocessor 50 via the third transport rollers 13, the fourth transportrollers 14, and the fifth transport rollers 15 provided on the secondtransport path R2.

FIGS. 13A through 13D illustrate a procedure of saddle-stitch-bindingindirect output processing in the post-processing apparatus 2 of thesecond exemplary embodiment.

FIG. 13A illustrates a first state (the state in which a sheet P is fedfrom the image forming apparatus 1 via the feed-in path R0) of thesaddle-stitch-binding indirect output processing. In the first state,the first gate 21 is located at a position at which the feed-in path R0and the second transport path R2 are connected (the feed-in path R0 andthe first transport path R1 are disconnected), the third gate 23 islocated at a position at which the upstream side of the second transportpath R2 and the fourth transport path R4 are connected (the upstreamside and the downstream side of the second transport path R2 aredisconnected), and the fourth gate 24 is located at a position at whichthe upstream side and the downstream side of the second transport pathR2 are connected (the upstream side of the second transport path R2 andthe fifth transport path R5 are disconnected). In thesaddle-stitch-binding indirect output processing, the sheet P does notpass through the mounting position of the second gate 22.

In the first state, the sheet P fed from the image forming apparatus 1through the feed-in path R0 is input into the fourth transport path R4via the second transport path R2 through the third transport rollers 13provided on the second transport path R2 and the eighth transportrollers 18 provided on the fourth transport path R4, as indicated by thethick arrow in FIG. 13A. Then, after the rear end of the sheet P in thetransport direction passes through the portion at which the fourthtransport path R4 branches off from the second transport path R2, therotation of the eighth transport rollers 18 is stopped.

FIG. 13B illustrates a second state (the state in which the sheet Premains in the fourth transport path R4) after the first state shown inFIG. 13A in the saddle-stitch-binding indirect output processing. In thesecond state, both of the forward and rear ends of the sheet P in thetransport direction are located in the fourth transport path R4.

FIG. 13C illustrates a third state (the state in which the sheet P isfed out from the fourth transport path R4) after the second state shownin FIG. 13B in the saddle-stitch-binding indirect output processing.

In the third state, the eighth transport rollers 18 provided on thefourth transport path R4 are rotated in the same direction as that whenthe sheet P is fed into the post-processing apparatus 2, thereby causingthe sheet P to be transported in the same direction as that when thesheet P is fed into the post-processing apparatus 2. Accordingly, thesheet P is transported from the fourth transport path R4 to the secondtransport path R2, as indicated by the thick arrow in FIG. 13C, and isoutput to the saddle stitch binding processor 50 via the fifth transportrollers 15 provided on the second transport path R2.

In the example shown in FIG. 13C, the sheet P which is transported tothe fourth transport path R4 is singly output to the saddle stitchbinding processor 50. However, as shown in FIG. 13D, in accordance withthe timing at which the next sheet P (indicated by the long dasheddotted arrow) is subjected to saddle-stitch-binding direct outputprocessing through the second transport path R2, the sheet P (indicatedby the solid arrow in FIG. 13D) transported via the fourth transportpath R4 may be output to the saddle stitch binding processor 50. In thiscase, two sheets P are output to the saddle stitch binding processor 50in the state in which they are superposed on each other.

The procedure of the end-binding processing in the post-processingapparatus 2 of the second exemplary embodiment is the same as that ofthe first exemplary embodiment discussed with reference to FIG. 7, andthe procedure of the saddle stitch bookbinding processing in thepost-processing apparatus 2 of the second exemplary embodiment is thesame as that of the first exemplary embodiment discussed with referenceto FIG. 8. Thus, a detailed explanation thereof will be omitted.

In the second exemplary embodiment, as well as in the first exemplaryembodiment, the fourth transport path R4 is used both as a transportpath on which a sheet P temporarily remains in a standby state duringthe end-binding indirect output processing before being output to theend binding processor 30 and a transport path on which a sheet Ptemporarily remains in a standby state during the saddle-stitch-bindingindirect output processing before being output to the saddle stitchbinding processor 50. Thus, the size of the post-processing apparatus 2does not have to be increased, unlike a case in which a transport pathon which a sheet P remains in a standby state during the end-bindingindirect output processing and that during the saddle-stitch-bindingindirect output processing are separately provided.

Additionally, in the second exemplary embodiment, as well as in thefirst exemplary embodiment, the fourth transport path R4 is used bothfor end binding processing and saddle stitch bookbinding processing.Accordingly, the length of a sheet P (in the transport direction)subjected to the end binding processing is equal to that subjected tothe saddle stitch bookbinding processing. That is, in the end bindingprocessing, a sheet P having the same length as that of a sheet Psubjected to the saddle stitch bookbinding processing may be used.

In the first and second exemplary embodiments, end binding processing,which serves as first processing, is performed on a sheet bundle, andsaddle stitch bookbinding processing (saddle stitch binding processingand two-folding processing), which serves as second processing, isperformed on a sheet bundle. However, processing performed on a sheetbundle is not restricted to end binding processing and saddle stitchbinding processing (and two-folding processing). Another type ofprocessing, such as punching processing for punching a sheet bundle orthree-folding processing (C-folding or Z-folding) for folding a bundlesheet into three, may be performed.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

What is claimed is:
 1. A sheet processing apparatus comprising: afeed-in path through which a sheet is fed; a first transport paththrough which a sheet fed from the feed-in path is transported; a firstprocessor that sequentially stacks sheets output from the firsttransport path and that performs first processing on a bundle of stackedsheets; a second transport path through which a sheet fed from thefeed-in path is transported; a second processor that sequentially stackssheets output from the second transport path and that performs secondprocessing, which is different from the first processing, on a bundle ofstacked sheets; and a common retention path that causes a sheet fedthrough the feed-in path to temporarily remain on the common retentionpath and that causes the sheet temporarily remaining on the commonretention path to be output to one of the first transport path and thesecond transport path.
 2. The sheet processing apparatus according toclaim 1, further comprising: a connection transport path that branchesoff from the common retention path or the second transport path and thatjoins the first transport path.
 3. The sheet processing apparatusaccording to claim 1, wherein: the common retention path is positionedat a portion interposed between the first transport path and the secondtransport path; and a direction in which a sheet remaining on the commonretention path is transported to the first transport path is opposite toa direction in which a sheet remaining on the common retention path istransported to the second transport path.
 4. The sheet processingapparatus according to claim 2, wherein: the common retention path ispositioned at a portion interposed between the first transport path andthe second transport path; and a direction in which a sheet remaining onthe common retention path is transported to the first transport path isopposite to a direction in which a sheet remaining on the commonretention path is transported to the second transport path.
 5. An imageforming system comprising: an image forming apparatus that forms animage on a sheet; and a post-processing apparatus that performspost-processing on a sheet on which an image is formed by the imageforming apparatus, the post-processing apparatus including a feed-inpath through which a sheet is fed, a first transport path through whicha sheet fed from the feed-in path is transported, a first processor thatsequentially stacks sheets output from the first transport path and thatperforms first processing on a bundle of stacked sheets, a secondtransport path through which a sheet fed from the feed-in path istransported, a second processor that sequentially stacks sheets outputfrom the second transport path and that performs second processing,which is different from the first processing, on a bundle of stackedsheets, and a common retention path that causes a sheet fed through thefeed-in path to temporarily remain on the common retention path and thatcauses the sheet temporarily remaining on the common retention path tobe output to one of the first transport path and the second transportpath.